<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script type="text/javascript">

    function getHeaders(){
        var req = new XMLHttpRequest();
        req.open('GET', document.location.href, false);
        req.send(null);
        var headerArr = req.getAllResponseHeaders().split('\n');
        var headers = {};
        headerArr.forEach(item=>{
            if(item!==''){
                var index = item.indexOf(':');
                var key = item.slice(0,index);
                var value = item.slice(index+1).trim();
                headers[key] = value
            }

        })
        return headers
    }



function loadXMLDoc()
{
//var headers = getHeaders();
//console.log(headers);
var xmlhttp;
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
    }
  }
//设置headers头部

xmlhttp.open("GET","/grayRequest",true);
//xmlhttp.setRequestHeader('version',headers['version']);
xmlhttp.send();
}
</script>
</head>
<body>

<div id="myDiv"><h2>Let AJAX change this text</h2></div>
<button type="button" onclick="loadXMLDoc()">longPulling</button>

</body>
</html>